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Re: Effects of the size of toroids
> Original Poster: "Neal Whitaker" <NDWhitaker-at-Worldnet.att-dot-net>
Hi Neal,
Good question.
> <snip>My question is as follows: How does the surface area or size of the
> toroid affect the ouput of the coil.?
The size of the coil as far as how wide it is compared to the secondary or
how narrow the cross section is can change between coils yet have the same
capacitance. BTW, the figure you gave for 11.035" x 8" is 6.8pF, not 20pF.
However, your 1" x 14.3" was 20pF. The formula I use is Ctop =
((1+(.2781-d2/d1))x2.8)*(sqrt(2pi^2(d1-d2)*(d2/2)/4pi))
where d1 = outer diameter of toroid
where d2 = diameter of the cross section
Regardless, I do believe physical size does play a role with field control
of the spark channel. The radius of curvature (ROC) determines in part the
breakout potential of the toroid. The capacitance is the capacitance the
toroid can be charged to, however, the imperfections on the surface will
degrade that level of charge. Also, a narrow cross section vs. a wide cross
section will change this (some). A wide cross section is usually smaller and
deeper into the field of the coil. I don't have a good answer for you
because there are coils that run excellent with fat toroids and with narrow
but wide toroids. I do believe charge potential based on the capacity the
toroid is essential for optimum spark length, but we can't think only about
the capacity. The size of the toroid is also a factor regarding it's
geometry to the rest of the coil both mechanically and electrically. Some
people actually use the secondary as a "size" guide for the toroid. I
personally use a factor of 2.8 x Cself to find a toroid capacitance. I then
try to find that capacitance based on 2.5 x the o.d. of the secondary. This
get's me into the ball park, but may not be perfect for everyone's coil.
Bart